Extend AnrController to improve ANR dialog controls

I5431d22bd8faa75e3deab1960a2f721784f98d42 introduced a basic ANR
controller interface to allow services within the system_server
register to delay the ANR dialog during an ANR.

Some limitations of that interface were:
1. No way to cancel the ANR dialog if the normal function of the app
resumed during the delay
2. The AnrController#getAnrDelay method was serving a dual purpose of
returning the delay and showing an interim UI
3. The interim UI might live within an updatable mainline
module, hence forcing the system_server to block (asynchronously) on
the 'getter' for the binder call.

This cl addresses the problems above with the following new
methods on AnrController: onAnrDelayStarted and onAnrDelayCompleted.

These methods provide a clearer interface for the following reasons:
1. onAnrDelayCompleted will return a boolean value to indicate whether
the ANR dialog should still be shown after the delay
2. onAnrDelayStarted is a more explicit API that can be called
asynchronously without blocking to notify external components to show
an interim UI
3. The existing getAnrDelay can be handled completely within the
system_server without blocking on an external binder call

See design (2a) in go/transcoding-anrs for more details

Test: Manual
Bug: 170486601
Change-Id: Idb9190a0e6014ce64bf1412c26f6ae03f97e922d